What is standard practice with regard to vibration isolation on ducted heat pumps?

[ezheidtmann]

Hi! I have a contractor installing a ground-source heat pump with a ducted air handler and I'm concerned about the noise level I'm experiencing in the living space.

There is some noise being transmitted through the ducts, both the return and the supply registers. The installation manual for the heat pump recommends a flexible connector between the air handler and the ducting. I've seen these in other installations: a short (1-2 inch) collar of rubber (or fabric?) that serves to reduce transmission of vibration from the air handler to the connected ductwork.

My installer has said that these type of isolation measures are not typical in modern installations. Is this true? What are typical and standard measures that are used to reduce noise transmission?

Thanks for your wisdom!

-Evan

More details about my installation: The heat pump is a Bosch SM-036. This is a retrofit on an old house, but all duct work connecting to the air handler is new as of this installation.

[Bazooka Joe]

If the installation manual recommends installing a canvas connector(s) , then yes I would have the Installing Contractor have them installed . Installing them would be a good start, chances are the Contractor is balking at doing this as it would reduce their profit margin. The instructions may or may not mention to loosen the four compressor bolts a little. Sometimes units are shipped with the compressor bolts tight, and they need to be backed off ( per manufacture instructions ) a little so the rubber grommets that the compressor sits on can do their job.

Possible your duct work is not sized correctly also causing noises, does your duct ( if metal ) flex on start up/shutdown and bangs when the blower comes on/off? Do you get whistling noises from your supply registers.

Water source issues with the loop .

If you run the blower only, does the noise dissipate?

Does the noise sound the same in cooling mode as in heating mode?

[ezheidtmann]

Good questions! Thanks for your time. The manual shows a "typical installation" that includes flex connections but doesn't elaborate. I will ask the installer to add them.

I don't know how to evaluate the loop health, but the installer says it is in good shape. There is significantly less noise in cooling mode, but the character is the same: a hum mixed with some buzzing, sometimes rhythmic. I don't have whistling noises, flex, or banging in the duct work. The blower alone is definitely audible from registers, but is a much lower volume than the compressor noises.

[beshvac]

Personally, I hate flex connections and have not installed on modern equipment in the last 8 years. They are almost impossible to seal for no leakage.

So if you were to call my company and insist on something not necessary, I would tell you I would be out to see what the real issue is.

[shavedneon]

We haven't be able to install flex connectors legally indoors for many years (15?) if I remember correctly it was a smoke/flame spread issue.
I believe the code would be international.

[Bazooka Joe]

I looked up your unit and some of the things mentioned was if the unit is installed in a vertical position your suppose to install a vibration absorbing pad under the unit, along with the using flexible connectors in both the supply and return duct, and not to hook air duct directly to blower outlet. To Use flexible water lines as ridged tubing causes vibration. If you have any concerns about codes, I would go to the local fire Marshall or town hall building department in regards to your flex connectors. I certainly could be wrong but I would think that would apply more for combustion furnaces of any type, corn, wood,pellets,oil,gas etc.

From Bosch SM unit manual

A flexible connector is recommended for supply and return air duct connections on metal duct systems. All metal ducting should be insulated with a minimum of one inch duct insulation to avoid heat loss or gain and prevent condensate forming during the cooling operation. Application of the unit to uninsulated duct work is not recommended as the unit's performance will be adversely affected." Do not connect discharge ducts directly to the blower outlet " . The factory provided air filter must be removed when using a filter back return air grill. The factory filter should be left in place on a free return system.If the unit will be installed in a new installation which includes new duct work, the installation should be designed using current ASHRAE procedures for duct sizing. The duct system and all diffusers should be sized to handle the designed air flow quietly. To maximize sound attenuation of the unit blower, the supply and return air plenums should be insulated.There should be no direct straight air path thru the return air grille into the heat pump. The return air inlet to the heat pump must have at least one 90 degree turn away from the space return air grille.

If you can find the following quote in your install guide " Do not connect discharge duct directly to the blower outlet" ask them was that means.

Water Furnace, another large manufacture of GeoTherm Heat Pumps , shows the vibration absorber pad on vertical install unit along with flexible connectors on both sides. Their horizontal installs shows specific mounting guides along with Bosch and again the flexible connectors on both sides of the unit

[ezheidtmann]

Bazooka Joey -- Thanks for finding that paragraph! I had missed it on my first reading. I will ask my installer to explain that line.

beshvac and shavedneon -- Thanks for weighing in. Since you don't do these flex connectors, what other techniques do you employ for customers that complain of excessive noise being transmitted through metal ducting?

Thanks all!

Evan

[beshvac]

Bazooka Joey -- Thanks for finding that paragraph! I had missed it on my first reading. I will ask my installer to explain that line.

beshvac and shavedneon -- Thanks for weighing in. Since you don't do these flex connectors, what other techniques do you employ for customers that complain of excessive noise being transmitted through metal ducting?

Thanks all!

Evan

Nothing, because its not needed just recommended. And if there is no noise, then I won't go back and put one in. If there is noise, I will find out why its shaking that much.

For example, haire equipment that I have come across- I would always put in vibration isolation because that crap will always shake